TST: also run flake8 on Python 2.7

[tswast]

Closes #54.

[tswast]

Oops, duplicate of #54

[codecov-io]

Codecov Report

Merging #55 into master will decrease coverage by 44.87%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #55       +/-   ##
===========================================
- Coverage   74.18%   29.31%   -44.88%     
===========================================
  Files           4        4               
  Lines        1511     1511               
===========================================
- Hits         1121      443      -678     
- Misses        390     1068      +678

Impacted Files	Coverage Δ
pandas_gbq/gbq.py	16.08% <0%> (-63.14%)	⬇️
pandas_gbq/tests/test_gbq.py	31.88% <0%> (-49.94%)	⬇️

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